From what I can see it appears to be bumblefoot. That looks like a good size one, but it may be the picture (sometimes closer objects appear larger).
I am very cautious about cutting into feet - actually have not tried that. I have been able to catch bumblefoot very early so that all I have had to do is clean and apply triple antibiotic. After a few weeks, it's gone.
The insides, from what I have read, are not watery, or easy to drain.
Perhaps some people with more experience with more advanced cases can advise you.
Could you check with a vet? Cutting open a foot increases the risk of infection. But leaving a bumble alone can also lead to more trouble. I just don't know if it is far enough along to justify cutting it open, or if some epsom salt soaks and antibiotic would do it. In any case, it takes a while. Try not to lose patience.
Poor little duck.
Until you are sure, I would start with some soaking and cleaning and antibiotic ointment. You don't want the duck to drink the epsom salts water, so a compress would make more sense.
